Seasonal Stratospheric Photochemistry on Uranus and Neptune.

A time-variable 1D photochemical model is used to study the distribution of stratospheric hydrocarbons as a function of altitude, latitude, and season on Uranus and Neptune. The results for Neptune indicate that in the absence of stratospheric circulation or other meridional transport processes, the hydrocarbon abundances exhibit strong seasonal and meridional variations in the upper stratosphere, but that these variations become increasingly damped with depth due to increasing dynamical and chemical time scales. At high altitudes, hydrocarbon mixing ratios are typically largest where the solar insolation is the greatest, leading to strong hemispheric dichotomies between the summer-to-fall hemisphere and winter-to-spring hemisphere. At mbar pressures and deeper, slower chemistry and diffusion lead to latitude variations that become more symmetric about the equator. On Uranus, the stagnant, poorly mixed stratosphere confines methane and its photochemical products to higher pressures, where chemistry and diffusion time scales remain large. Seasonal variations in hydrocarbons are therefore predicted to be more muted on Uranus, despite the planet's very large obliquity. Radiative-transfer simulations demonstrate that latitude variations in hydrocarbons on both planets are potentially observable with future JWST mid-infrared spectral imaging. Our seasonal model predictions for Neptune compare well with retrieved C2H2 and C2H6 abundances from spatially resolved ground-based observations (no such observations currently exist for Uranus), suggesting that stratospheric circulation - which was not included in these models - may have little influence on the large-scale meridional hydrocarbon distributions on Neptune, unlike the situation on Jupiter and Saturn.

Photochemistry on Pluto: part II HCN and nitrogen isotope fractionation.

We have converted our Titan one-dimensional photochemical model to simulate the photo- chemistry of Pluto's atmosphere and include condensation and aerosol trapping in the model. We find that condensation and aerosol trapping are important processes in producing the HCN altitude profile observed by the Atacama Large Millimeter Array (ALMA). The nitrogen iso- tope chemistry in Pluto's atmosphere does not appear to significantly fractionate the isotope ratio between N2 and HCN as occurs at Titan. However, our simulations only cover a brief period of time in a Pluto year, and thus only a brief portion of the solar forcing conditions that Pluto's atmosphere experiences. More work is needed to evaluate photochemical fractionation over a Pluto year. Condensation and aerosol trapping appear to have a major impact on the altitude profile of the isotope ratio in HCN. Since ALMA did not detect HC15N in Pluto's atmosphere, we conclude that condensation and aerosol trapping must be much more efficient for HC15N compared to HC14N. The large uncertainty in photochemical fractionation makes it difficult to use any potential current measurement of 14N/15N in N2 to determine the origin of Pluto's nitrogen. More work is needed to understand photochemical fractionation and to evaluate how condensation, sublimation and aerosol trapping will fractionate N2 and HCN.

Photochemistry on Pluto - I. Hydrocarbons and aerosols.

In light of the recent New Horizons flyby measurements, we present a coupled ion-neutral-photochemistry model developed for simulating the atmosphere of Pluto. Our model results closely match the observed density profiles of CH4, N2 and the C2 hydrocarbons in the altitude range where available New Horizons measurements are most accurate (above ~ 100-200 km). We found a high eddy coefficient of 106 cm2 s-1 from the surface to an altitude of 150 km, and 3 × 106 cm2 s-1 above 150 km for Pluto's atmosphere. Our results demonstrate that C2 hydrocarbons must stick to and be removed by aerosol particles in order to reproduce the C2 profiles observed by New Horizons. Incorporation into aerosols in Pluto's atmosphere is a significantly more effective process than condensation, and we found that condensation alone cannot account for the observed shape of the vertical profiles. We empirically determined the sticking efficiency of C2 hydrocarbons to aerosol particles as a function of altitude, and found that the sticking efficiency of C2 hydrocarbons is inversely related to the aerosol surface area. Aerosols must harden and become less sticky as they age in Pluto's atmosphere. Such hardening with ageing is both necessary and sufficient to explain the vertical profiles of C2 hydrocarbons in Pluto's atmosphere. This result is in agreement with the fundamental idea of aerosols hardening as they age, as proposed for Titan's aerosols.